from tkinter import *
from tkinter.ttk import *


def selected(evt):
    print('当前选中的值是:', cb.get())


win = Tk()
win.geometry("300x300")
values = ["1", "2", "3", "4", "5"]

cb = Combobox(win, values=values)
cb.current(3)
cb.bind("<<ComboboxSelected>>", selected)
cb.pack()
win.mainloop()